This invention relates to metal removal from a titanium alloy article, and, more particularly, to the electrochemical machining of a titanium alloy article made of the titanium alloy Ti 17.
Electrochemical machining is a technique for removing metal from a metallic article, and in concept is roughly the inverse of electroplating. In electrochemical machining (or "ECM"), a workpiece is made the anode of an electrolytic cell having a cathode in the shape of a metalworking tool and an electrolyte. The electrolyte is a fluid, typically an aqueous solution of various ionic species, that carries the electrical current of the cell between the anode and the cathode. When an electric current is passed through the cell, metal is dissolved from the workpiece in a pattern dictated by the shape of the cathode.
Electrochemical machining is usually conducted with high electrical current densities of up to several hundred amperes per square inch. The spacing between the anode and the cathode is small, usually 0.1 to 1.5 millimeters, or even less. The electrolyte is pumped through the space between the anode and the cathode at high flow rates and pressures, to carry away heat, gas bubbles, and dissolved metallic species from the anode.
Under these conditions, metal removal is fast but must also be uniform. For metallic articles to be used in precision applications such as aerospace hardware, the surface finish of the article must be very smooth, on the order of 25 microinches or less variation. The surface must be free of macroscopic defects such as ridges, pitting, flow patterns, etched appearance, or visible patterns.
Some metals are particularly difficult to electrochemically machine at high rates of metal removal, while attaining a good surface finish. One commercially important example is titanium and titanium alloys (generally termed "titanium" herein). Titanium forms an oxide surface layer, both in normal exposure and during electrochemical machining. The oxide passive coating must be removed, requiring the use of chemically aggressive electrolytes. Although a number of electrolytes can be used to remove titanium from an article by electrochemical machining, these known electrolytes often lead to surface irregularities of the type discussed. Where there are ridges, pits, and other defects formed on the electrochemically machined surface, expensive post-machining polishing and/or repairs may be necessary.
There is a need for an improved approach to the electrochemical machining of titanium alloys, and in particular the titanium alloy Ti 17. Different titanium alloys respond differently to ECM, and therefore an electrolyte that is operable for one titanium alloy would typically not be operable for another titanium alloy. Thus, there is needed an electrolyte that is custom tailored for ECM of Ti 17 alloy. The present invention fulfills this need, and further provides related advantages.